When user equipment (UE or MS) attaches to a cellular network it connects to a core network entity that may be a member of a pool of core network entities, e.g., for WCDMA or LTE case a mobility management device (MME), e.g., a Serving GPRS Support Node (SGSN) or Mobility Management Entity (MME). Standardization protocols enable movement between different SGSNs of subscribers performing Routing Area Update (RAU) and at attachment, but also the means necessary to move remaining subscribers not performing any of these operations. The remaining subscribers are those that are immobile enough to not perform a RAU and at the same time are running payload frequently enough to not execute the periodic RAU procedure. Movement of these subscribers may be performed by executing a detach procedure, with reattach required.
Redistribution of MSs in an SGSN Pool is described in 3GPP TS 23.236 and for an MME Pool in 3GPP 23.401. The Re-distribution of MSs is initiated via an O&M command in the CN node, which needs to be off-loaded.
In a first phase (a couple of Periodic LU/RAU periods long), MSs performing RAU or Attachment are moved to other SGSNs in the pool. When the SGSN receives the Routing Area Update or Attach request, it returns a new P-TMSI with a null-NRI, and a non-broadcast RAI in the accept message.
In the PS domain, a new Routing Area Update is triggered by setting the periodic routing area update timer to a sufficiently low value (recommended value is 4 seconds) in the accept message. The MS will shortly thereafter send a new Routing Area Update that the RAN node then will route to a new SGSN due to the presence of a null-NRI.
In a second phase, the SGSN requests all MSs trying to set up PDP Contexts to detach and reattach. When they reattach, the SGSN moves the MSs as in the first phase described above.
A third phase includes scanning through the remaining MSs and initiating a move of the MSs to other SGSNs. In the PS domain MSs are requested to detach and reattach, which will cause them to be moved
MSs being moved from one SGSN are stopped from registering to the same SGSN again by an O&M command in BSCs and RNCs connected to the pool. MSs moving into a pool area may also be stopped from registering into a SGSN being off-loaded in the same manner.
According to 3GPP 23.236, in an SGSN Pool, there are situations where a network operator will wish to remove load from one SGSN node in an orderly manner (e.g., to perform scheduled maintenance, or, to perform load re-distribution to avoid overload) with minimal impact to end users and/or without placing additional load on other entities. The re-distribution procedure does not require any new functionality in the terminal, that is, all terminals can be moved.
However, when an MS is in PMM-CONNECT (i.e., is active), it will not perform periodic RAUs or attachments. With the solution suggested in 23.236, if operators want to move the MSs in PMM-CONNECT with PDP, the current PS service will be interrupted. The MS has to re-establish the service in the target SGSN of the move.
Such interruptions are unacceptable for the MS doing uninterruptible service, such as VoIP, FTP, streaming, and so on. A procedure of moving an MS in PMM-CONNECT with minimized packet loss is therefore a highly wanted feature.
Moreover, the existing solution has the following problems:                No standardized way or suggestion on how to move active subscribers in SGSN from one pool member to another without using the crude detach.        Payload interrupts, which could last many seconds during move operations between pool members, exist. This is valid for load re-balancing of both the SGSN Pool and the MME Pool.        The standardized way to perform load re-balancing between MMEs is based on distribution performed by the eNodeB. This means that without additional information the eNodeB may only perform random distribution of MSs based on the MME capacity weight and is not able to consider end-user behavior and its impact on the MME load.        
An arrangement and a method for moving active subscribers without significant service interruption are therefore highly desired.